Various technologies are useful for controlling emissions from motor-vehicle engine systems. A regenerable soot filter such as a diesel particulate filter (DPF) may be used to trap and oxidize soot. A lean nitrogen-oxide trap (LNT) may be used to trap and periodically reduce nitrogen oxides (NOx). A selective catalytic reduction (SCR) catalyst may be used to destroy NOx by reacting it with a nitrogenous reducing agent. Further, some of these technologies can be combined for added benefit.
In one example, U.S. Pat. No. 7,485,273 to Gandhi et al. describes an exhaust aftertreatment system for a motor vehicle, in which an SCR catalyst is coupled downstream of an LNT. In this configuration, the SCR catalyst traps ammonia that is released by the LNT during rich conditions. During subsequent lean conditions, NOx that fails to be reduced in the LNT is reduced by the trapped ammonia.
In another example, U.S. Patent Application Publication 2001/0032459 describes a soot filter in which an LNT catalyst is applied to a DPF substrate (a DPF/LNT). Unlike other soot filters, which must be regenerated by periodic exposure to high-temperature exhaust, the DPF/LNT is capable of continuous regeneration at temperatures greater than 250° C. It is believed that ‘active oxygen’ evolved in the catalyst by storage of NOx accelerates oxidation of the entrapped soot.
The inventors herein have recognized various synergies that can be achieve when coupling an SCR catalyst downstream of a DPF/LNT. In one embodiment, a method for controlling emissions from an engine is provided. The method comprises reducing trapped nitrogen oxides to ammonia on an LNT catalyst while concurrently oxidizing soot accumulated on the LNT catalyst. The method further comprises flowing the ammonia so formed to an SCR catalyst. In this manner NOx emissions are controlled more effectively than would be possible with an LNT alone. Moreover, soot is removed as well as NOx, and under some operating conditions, continuous, fuel-efficient regeneration of soot-trapping capacity is possible while still providing ammonia to the SCR. In one embodiment, reducing the trapped NOx includes flowing exhaust from the engine over the LNT catalyst. Here, at least one of an air-to-fuel ratio of the exhaust and a duration of said flowing may be adjusted in response to an amount of soot accumulated on the LNT catalyst. In this way, it is possible to provide the appropriate amount of reductant to the LNT, taking into account the soot collected thereon, which may also serve as a reductant for NOX.
It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description, which follows. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted herein.